1. Field of the Invention
The present invention relates to an image sensor. More particularly, it relates to an image sensor which senses an optical state of a device in real time and which has an automatic gain control (AGC) function for determining a time for storing charges.
2. Description of the Related Art
In image sensors usually used as an optical sensor, a plurality of picture element diodes are arranged in a regular pattern with a plurality of columns over the whole surface of a semiconductor substrate; and, between each picture element diode column, first transfer gates which are common to each picture element diode column, and shift registers in a horizontal direction (image sensors) which are also common to each column, are arranged. At one end of the registers in the horizontal direction, a second transfer gate which is common to each shift register in the horizontal direction and a shift register in vertical direction are arranged, and an amplifier is connected to one end of the shift register in the vertical direction.
An optical sensor using such image sensors, has only the function that the charges stored in the picture element diodes by an incidence of light are sent to the amplifier in sequence only by the shift registers, so as to output these variations of the intensity of light as electric signals. Such a device functions sufficiently when the intensity of the light is restricted to a limited range, such as in a facsimile device. However, when it is used, for example, in a camera, in which the dynamic range of the light signal is broad, the function is not properly effective because a light overflow state or too intense a light signal results in an excessive amount of light being received or a decrease in sensitivity results in an insufficient amount of light being received.